Diffraction effects of a tape covering the gaps of a panelled ‘compact range’ operating at millimetre waves

M. Philippakis, MSc, AMIEE C.G. Parini, PhD, MlEE

Indexing terms: Compact ranges, Diflraction, Millimetre waves, Tupe scattering

Abstract: Diffraction which arises at the inter- panel gap edges of a panelled reflector in a ‘compact antenna test range’ clutters the measured radiation pattern with the formation of spurious lobes. Here the approach of covering the gaps with a thin metallic tape is studied in terms of its effectiveness to suppress the spurious effects at frequencies mainly in the 180 GHz band. A ray GO/GTD like approach is employed. Diffraction from the tape edges is accounted for by a recently presented formulation. The conclusion of the analysis is that the tape can itself induce a spuri- ous lobe, the strength of which depends on factors such as tape thickness and width as well as its location on the reflector surface. In addition, the spurious lobe strength is affected by the edge mis- alignment of the gap which the tape covers as well as the possible pillowing of the tape over the gap. Experimental results are presented that agree with theoretical predictions. Spurious lobes with a level around -45dB have been observed in practice, providing an average figure of 13 dB suppression relative to the bare gap case.

1 Introduction

The panelled approach is the technique usually adopted for the construction of the reflector(s) of a compact antenna test range (CATR) to fulfil the requirements of high surface accuracy essential for millimetre wave oper- ation. In Reference 1 the GO/UTD technique was employed to study the effects of interpanel gap edge dif- fraction. The conclusions of this work were that at milli- metre waves gap edge diffraction can impair the CATR ‘quiet zone’ quality. When an antenna is tested in a panelled CATR with untreated gaps then spurious [obes are induced in its measured pattern. These lobes are centred at angles where the gaps are subtended by the test antenna main beam. The severity of the gap induced effects is determined primarily from the width of the gap and from the panel misalignment. The latter is the dominant factor when its magnitude exceeds the figure of 1/60. At 183 GHz, a typical measured value for the

0 IEE, 1994 Paper 102SH (Ell), received 2nd June 1993 The authors are with the Electronic Engineering Department, Queen Mary and Westfield College. London University, Mile End Road, London El 4NS, United Kingdom

114

strength of the spurious lobes was found to be around - 30 dB.

In an attempt to rectify this situation a thin metallic tape can be used to cover the gaps. Here we will study the effectiveness of this approach by considering the impact of the tape on the ‘quiet zone’ field quality as well as the severity of the induced disturbances on the radi- ation pattern of a representative test antenna at milli- metre waves.

The effects of using tape in CATR applications was initially studied in References 2 and 3. In Reference 2 the authors have considered that the tape lays along the surface of an offset parabolic cylinder (2D geometry) with no gap underneath. The field scattered by the tape was found by straight forward application of the method of moments (MM). Examples of tape scattering were given, mainly at an operating frequency of 1 GHz, due to the limitations of MM in terms of the computer resources required when calculations at much higher frequencies are performed. As intuitively expected, tape scattering at these frequencies can be extremely weak. For example, at 1 GHz a tape 80 pm thick and 25.4 mm wide fixed on a CATR reflector will produce a peak scattered field with strength - 84 dB at a distance 6 m away. This value is expressed relative to the field produced at the same point from the CATR when the tape is absent, based on geo- metrical optics considerations. The authors in Reference 2 have also derived a formula expressing the peak tape scattering field at a given distance as a function of the geometrical parameters of the tape, by extrapolating their calculations.

The analysis in Reference 3 makes similar initial assumptions as in Reference 2, although the target is the estimation of the errors induced on the pattern of a test antenna. Scattered fields can also be found as an inter- mediate step. The tape is assumed to produce a constant phase error. The resulting perturbation in the CATRs plane wave spectrum (PWS) can be found by utilising the associated Fourier transform relations. Subsequently the coupling between a test antenna and the CATR is formu- lated in terms of the corresponding PWS. The term associated with the existence of the tape is extracted in the form of a residual coupling integral (RCI). For mechanically small, although highly directive, millimetre

The authors would like to thank Prof. A.D. Olver and Dr. Hai Zhou for their useful comments and suggestions. The assistance of J. Dupuy during the measurements is greatly appreciated.

I E E Proc.-Mirrow. Antennas Propag., Vol. 141, No. 2, April 1994

wave test antennas, asymptotic evaluation of the RCI reveals that the tape can induce a spurious lobe in the measured pattern, centred at the angle where the test

\ \

Fig. 0

1A Tflpe couering an inlerpanel gap

, \

LP

antenna main beam 'looks' at the tape. An expression for the strength of this spurious lobe as function of the tape's geometry and the distance between tape and test antenna, is presented. The derived expression for the peak scattered field agrees, in the low frequency limit, with that derived in Reference 2.

Both the approaches discussed above have some lim- itations. First, the MM procedure used in Reference 2 makes the approach unattractive at millimetre wave fre- quencies due to the excessive computational resources required. The approach in Reference 3, on the other hand, is unable to handle the effects arising due to pos- sible deformation (pillowing) of the portion of the tape which is above the gap, or edge misalignment of the interpanel gap which it covers (Fig. 1A). Both these factors are important in establishing the effectiveness of tape treatment at millimetre waves.

Here we assess the effects due to thc tape by utilising a GOjUTD ray technique as a tool for field computations. Apart from the ability to evaluate 'quiet zone' character- istics we are also focusing our attention on the effects induced on the radiation pattern of a representative test antenna. This is felt important on the grounds that it is not always obvious how to directly relate the effects of quiet zone degradations on the pattern of a given test antenna when measured in the CATR.

Test antennas with moderate physical dimensions (-1 m) can be quite large electrical structures at fre- quencies approaching 200 GHz. This fact prevents the full three-dimensional investigation of the subject due to the huge computational effort involved. Here a two- dimensional model has been studied, in common with previously discussed works. Although full polarisation

I E E Proc-Microw. Antennas Propag., Vol. 141, No. 2, April I994

information is sacrificed, that related to the main polari- sation used are to a great extend preserved, and the potential of valuable physical insight is provided.

front view

/ \ plan

ef lector \

\ test antenna

z feed \ b

Fig. 1 B Q M W compacl untennu test range

The fields produced by the CATR are calculated as a superposition of the geometrical optics (GO) field plus diffraction contributions. The latter can emanate from various diffraction centres such as the outer reflector edges, possibly uncovered interpanel gap edges, plus the edges of the tape (E, E' in Fig. 1A) as well as the points of the tape directly above the edges of the gap which it covers (G, G' in Fig. 1.4).

Diffracted fields produced by points such as E, E' can be calculated by utilising the ray methods presented in References 4 and 5. These methods are similar since they are both based on extracting an edge diffraction contri- bution from the exact solution of the canonical problem of a halfplane over an infinite ground plane using the Wiener Hopf method. The method presented in Refer- ence 4 is used here as it is simpler and faster to apply, and at the same time predictions based on this method are in very good agreement both with experimental data as well as calculations based on MM.

Diffracted fields arising from centres such as G, G are handled within the UTD wedge framework.

The theoretical modelling of the pattern measurement used here is similar to that presented in Reference 1. This formulation takes into account the fact that when the test antenna is placed in the CATR field its effective aperture distribution is a function of the intrinsic aperture dis- tribution of the test antenna and the CATR field values projected into that aperture. The relevant expression given in Reference 1 ignores the impact of the evanescent

115

components of the fields involved. This is of course entirely justifiable when dealing with electrically large structures which are separated by electrically large dis- tances.

Millimetre wave operation at 90 and 180 GHz will be studied here. Most of the examples presented are based on the geometrical dimensions of the QMW CATR [6] since it represents a typical example of a medium size range. The plan and front view of this facility as well as the reflector panel segmentation can be seen in Fig. 1B. Test antennas with 250 mm or 10M)mm aperture diam- eters will be used in the simulated pattern measurements, since practical antennas operating at these frequencies are likely to have dimensions falling within these limits. Test apertures field distribution belonging to the Taylor line source family [lo] are used. With this aperture dis- tribution we have the freedom of selecting the off bore- sight test antenna radiation level as low as desired. in order to study the tape effects without the danger that these will be masked under the test antenna’s sidelobe structure.

2 Theoretical considerations

2.1 Fields scattered due to the presence of a metallic tape perfectly fixed on a flat ground plane

In a recent letter [4] an asymptotic method for solving the above mentioned problem has been given, plane wave illumination having been assumed (Fig. 2a). In this

tape

,*

b H

Fig. 2 Equivalencesfor tupr edge diflraction

section the method is outlined and further evidence of its validity is presented.

At high frequencies diffraction is a localised phenom- enon arising in the vicinity of distinct centres. In the case

116

studied here these centres are the two edges of the tape. For electrically wide tapes the amount of diffraction caused due to one of these edges is assumed to be unaf- fected by the presence of the other. The problem then is reduced to that of diffraction from a step representing each edge (Fig. 26). Superimposing the appropriate con- tributions from the two steps plus the proper GO field will result in the total tape scattered field. To find the step diffraction field we further assume that the vertical walls of the tape can be ignored (Fig. 24. When the tape is electrically thin calculations based on this assumption agree very well with experimental data and MM calcu- lations. The solution of the new problem (Fig. 2 4 can be found analytically by taking advantage of the fact that there exist solutions of the semiinfinite parallel plate canonical problem [7, 81. The solution proceeds by assuming even or odd excitation of the parallel plate for the hard (H) case or the soft (E) case, respectively. This type of excitation has been chosen in order to satisfy the boundary conditions along the infinite ground plane at x = 0 of the original step problem. Uniform asymptotic expansion of the appropriately superimposed spectral solution of the parallel plate problem [4] yields the fol- lowing expression for the tape edge diffracted field at far distances

e ~ P r u ; ,“ ,c~P) = K(t; e, e,p,, de, e,) ~ uincid=yo) ( 1 )

JC.1 where

DE, = ordinary UTD diffraction coefficient [9] for an isolated halfplane having its edge along the y axis and extending to - CO along the z axis.

K = proximity factor given by the following expres- sion

sin (kt sin 8) cos (0/2) kt sin K + ( k cos e) K(t; 8, e,) = -4jkt

(2) sin (kt sin 8,) cos (8,/2) kt sin 0, G + ( k cos e,)

The auxiliary function G+(a) arises in the Wiener Hopf solution of the parallel plate problem and is given as follows

x exp [ - (1 - c + In, (z) - $)I exp [” In + (a - “)I

?I y - ( a - k )

x p = fi 1 (I - k ) e x p ( - E )

with

y = ,/(a’ - k’) and y p = J[@n/t)’ - k 2 ] (3b)

and C = 0.57721 . . . the Euler constant. The product KD,, is called the ‘tape edge diffraction

coefficient’. Time dependence exp (+jhfS), where 7 is time, has been assumed and suppressed. A single symbol U has been chosen to represent the field { E y or H , } for the two distinct cases arising in the two-dimensional problems.

We have further assumed that when the illumination is cylindrical and/or the tape follows the curved surface of

IEE Proc.-Microw. Antennas Propag., Vol. 141, No. 2, April I994

parabolic reflector, over which it is fixed, the tape edge diffraction coefficient retains the form KD,. H , where now the coefficient D E , H is modified according to the new illumination type as well as to the curvature of the half- plane in the standard UTD way. The effects of the illumi- nation type and reflector curvature are ignored in the factor K.

From the GO field point of view the tape is considered as a 'phase shifting surface' of zero thickness but with reflection coefficient

@'E = exp ( - Zjktrii,) (4) where

ri = outwards looking normal at the reflection point s , = a unit vector along the direction of the incoming

Initial experimental verification of the technique present- ed has been reported in Reference 4. A tape fixed on a conducting ground plane was illuminated by a distant source operating in the 90 GHz band. The field scattered from the tape was sensed with a probe mounted on a linear scanner that measured the transverse field strength this was then compared with that theoretically predicted. E-type polarisation was used and excellent agreement between theory and experiment was observed.

Here the following experimental philosophy will be adopted. As it stated before, a spurious lobe will be induced in the measured test antenna pattern if a thin tape is fixed on the surface of the CATR reflector. Milli- metre wave highly directive antennas only exhibit active pattern structure within a small angular region around the boresight direction. This means that for a spurious lobe emerging outside that region its strength can be measured without significant distortion to a pattern level as low as ~ 50 dB. Thus the experimental technique used was to fix a tape on the surface of the CATRs reflector, and then measure the radiation pattern of a directive test antenna. The level of the spurious lobe induced between t h s measured result and the data based on simulated measurements were then compared. The test antenna used was a 200mm aperture, 90" offset reflector oper- ating at 186.0GHz. The measurement geometry and arrangement is shown in (Fig. 1B). The test antenna is positioned so that it is facing the middle of panel 11. In the simulation a test antenna with aperture distribution producing very low sidelobe activity in the region where the spurious lobe is expected to emerge is required and apertures belonging to the Taylor family [9] with n = 10 and SLL = -SO or -55 dB were used. The position of the tape is located mathematically with the aid of the parametric angle 0,. This is the angle where the tape's centre is subtended at the focal point of the CATR reflec- tor, measured from the axis vertex-focal point as seen in (Fig. 1B). The H-polarisation results can be seen in Figs. 3a and b for the cases where a tape with a width of 25.4 mm and thickness of 10 and 65 pm, respectively, was fixed on the surface of panel 12 along the y axis of the QMW CATR.

Comparison can also be made between the present method and some published calculations [5] derived using the MM. In Reference 5 results are presented as linear scans of the tape scattered field normalised to the GO field that would exist in the same point if the tape was absent. The calculations using the present method uses the same geometry as in Reference 5. In particular the geometrical arrangement is similar to the one shown in (Fig. 1B) but the focal length is F = 7620 mm, and

wave

IEE Proc.-Microw. Antennas Propag., Vol. 141, No. 2, April 1994

-10

-20

-30-1

-40

-50

-60-

the upper and lower edges of the CATR reflector are determined from the parametric angles wuPpe, = 4 7 9 , wIowcr = 5". The scan is performed at a distance

O r

- 8

I

I

-

-

0 J

20 theta.degrees theta, degrees

a b

Fig. 3 Polarisation: H, wlsp = 47.5", range edge taper (ET) = - 14 dB __ Theory ~ ~ _ _ Experiment a t = LO pn, w = 25.4 mm b t = 65 pm, w = 25.4 mm

Spurious lobe due to a tapefixed on a panel

R = 12 190 mm. Uniform illumination is assumed. The H-polarisation case for 1 GHz operating frequencies is shown in Fig. 4. Very good agreement between the two - ; 0 -30k--!;;;;_:;;- ;,

- - _ _ -40 U

U U - _--- _ - - - _ - _ - - - 1 -50 a ' -60

-

1.83 2.44 3.05 3.66 4.27 4.88 5 4 9 distance. m

Fig. 4 H: wlarisatiaa, t I 000847 i,f= 1 GHz Tape width. w : D 0.339 1, b 0.677 I , c 1.350 1 ~ Present solution _ _ _ _ Method of moments

Comparison wifh the method ofmoments [SI

methods is evident, even for the cases where the electrical width of the tape is considerably less than one wave- length. The same level of agreement applies for the E- polarisation case. In the MM formulation the full features of the tape are taken into account, including the existence of the tape's vertical walls. Hence the good agreement with the MM makes the corresponding assumption of the present method justifiable.

2.2 Effects of the panel edge misalignment and the tape pillowing over the gap

When the tape is used to cover an interpanel gap two additional sources of scattering are associated with the misalignment rn of the gap edges under the tape and also the possible 'pillowing' of the tape over the gap (Fig. LA). By 'pillowing' we mean the curving of the tape's surface

117

over the gap. Pillowing can result due to the actual way that the tape is fixed or due to tape relaxation. A bulge h is introduced as a result.

In addition to the tape edge diffraction, the impact of edge misalignment and tape pillowing should also be included in the study of the effectiveness of tape treat- ment at millimetre wave frequencies. Both factors have been taken into account in the present 2D study. The assumption of uniform edge misalignment and tape pil- lowing along the gap's length is an idealised one. However, results so derived will serve as good indicators concerning the performance degradation of the CATR, under their influence.

It is very difficult to express the actual shape of the pillowing. Our practical experience, however, has shown that the circular arc shape (Fig. 1A) is near to the one observed. This is assumed in the analysis, since the curva- ture information is easily available then.

As a result of the tape deformation and edge misalign- ment the points of the tape above the gap edges become new diffraction centres. Diffraction arising from these new centres (C, G in Fig. 1A) is handled here using wedge UTD formulation. As can be seen in Fig. 1A the tape deformation and the possible combined action of panel edge misalignment creates a wedge, with curved faces, at the point G with an interior angle ci = (2 - n ) / z Similar arguments are applicable for G'. Previously the tape has been assumed as a phase shifting surface of zero thickness but with reflection coefficient given by eqn. 4. Adopting that convention we have heuristically modified the wedge diffraction coefficient to ensure continuity of the total field (GO + diffraction) across the o-face and the n-face reflection shadow boundaries (Fig. 1A). Fol- lowing the UTD notation of Reference 8, diffraction arising from a wedge with its edge at G can be expressed with the usage of the diffraction coefficient DE, as

with

PE! E = k exp (- Zjktri,,(G) ii)

and where ri,,"(G) = outwards looking normal of the o

F [ ] , E*(), I,:.: = UTD auxiliary functions and param-

Direct single diffraction only, arising at G, G' is going to be accounted for here. Ordinary multiple diffraction terms accounting for the mutual interaction between the wedges at G, G' will not be included. This assumption is made because the above approach accounting for the interaction is increasingly inaccurate for cases of near grazing incidence since then creeping wave mechanisms should be included. Even if multiple diffraction was an acceptable mechanism, it is going to contribute only a small part of the total double wedge diffracted field. Even this small contribution will have a questionable value in

118

and n face at G.

eters defined in Reference 9.

view of the underlying uncertainties implied by the ideal- istic nature of the model used. As a consequence these terms are neglected.

3 Effects of the tape in t he quiet zone quality of the CATR

The performance of a CATR is usually assessed by the quality of the field in the 'quiet zone' (test area). A bare interpanel gap can induce significant peak ripple in the near field [l] of the CATR especially when panel mis- alignment is present. A direct consequence of this is the cluttering of the measured pattern with spurious lobes of significant strength. Here the potential of tape treatment in restoring the field quality and hence reducing these lobes is considered.

Calculations and measurements presented will be based on the geometry of the QMW millimetre wave CATR. In most of the analytical examples to be present- ed a two panel CATR is studied, in order to focus our attention on the perturbation induced due to an individ- ual panel junction. For the same reason the CATR reflec- tor rim edge diffraction in most of the cases is disabled. Intutively very thin tapes will be appropriate for this application and hence tapes 10 and 40 pm thick will be mainly considered. Tapes much thinner than these will be very difficult to handle in practice.

Quiet zone scans at a frequency of 180 GHz can be seen in Fig. 5. A 10 or 40 mm wide tape is assumed to

3M)O 3400 3800 4200 4600 5cM3 distance,mm

Fig. 5 Polarisation: H, R = 6ooo mm Gap: width = 3 mm, positioned MO mm OR centre Tape: rn = b = 0 pm,range ET: - 3 dB Curves shifted for clarity a t=lOpm.m,=lOmm h t = I O p m , w = 4 0 m m e t = 4 0 p m m , w ; 4 0 m d 1 = 8 0 p m , w = l O p m e t = 80 Irm, w = 40 prn

Amplitude scan in quiet zone at I80 G H z

= wIap = 45")

stretch perfectly over a 3 mm interpanel gap which has no misalignment present. It can be clearly seen that when a very thin tape is used the maximum ripple has a much reduced value compared to its counterpart in the bare gap case, but most importantly the ripple in the central area is extremely small.

As it can be seen from Fig. 5 the peak ripple increases with an increase in width. The wider the tape the more directional its scattering pattern; this means that if the tape is sufficiently displaced from the central area of the reflector then the ripple induced there will not increase in proportion. However, the tape thickness has a direct effect on the ripple and as such is the most crucial param- eter.

The effects of the panel edge misalignment and tape pillowing over the gap are shown in Fig. 7, where a tape

I E E Proc.-Microw. Antennas Propag., Vol. 141, No. 2, April I994

10 pm thick and 40 mm wide is assumed. For compari- son purposes, the case of the bare gap with and without the same amount of edge misalignment can be seen in

11 0

_ _ -._ W P \ I .. -1 O t /-'

3200 3400 3600 3800 4000 4200 4400 4600 4800 d is tance,mm

Fig. 6 CATR with and without misalignment at 180 GHr Gap width = 3 mm, R = 6OOO mm Polarisation: H, mgap = 45", range ET: - 14 dB U Reference case: solid reflector with no gap h m = 0 p m c m = M ) p m

Quiet zone amplitude scan along AA' (Fig. I B ) in a 2-panel

11

. -3 -2t /A\ 2 . t / C \ / \

-9 3200 3400 3600 3800 4000 4200 4400 4600 4800

dis tance,mm Fig. 7

field quality at I80 GHz Polarisation: H Gap: width = 3 mm with mQmp = 45", R = 6Mo mm Tape: t = 10 pn, w = 40 mm, range ET: - 14 dB a m = h = 0 p m b m = M ) p m , b = O p m c m = M ) p m , b = L S O p n

Eflects of panel misalignment and tape pillowing on quiet zone

Fig. 6. Under the influence of pillowing and edge mis- alignment the field quality is gradually impaired to the point where the tape treated gap behaves in a similar way to the case where the gap has been left uncovered. In Fig. 8 the peak ripple is shown for a variety of conditions when a 10 pm thick, 40 mm wide tape is used to cover a 3 mm wide gap.

Finally, in Fig. 9 the measured quiet zone field at 90 GHz is shown and compared with theoretical calcu- lations. The scan direction was horizontal along the AA' axis (Fig. lB), and passed across the middle of panel 11. The gaps between panels 10-11 and 11-12 are about 2 mm wide and are covered with a 10 pm thick, 40 mm wide aluminium tape. In the calculations a three panel CATR has been used with diffraction from the outer edges of the reflector included to match the experimental situation. The calculations include a measured value of 40pm and 60pm for the edge misalignment between panels 10-1 1 and 11-12, respectively. The currently used CATR surface evaluation technique (twin theodolite triangulation) provides position information with an rms error of 60 pn. ln this respect there is room for better agreement between measurements and calculations if the

I E E Proc.-Microw. Antennas Propag., Vol. 141, No. 2, April 1994

misalignment factor was used as a free parameter varying between the permissible tolerance limits. During the experiment the interpanel gaps were filled with a silicone

0 b Fig. 8 lowing Tape at centre of reflector (olOp = 40") Polarisation: H Gap width: 3 mm Tape with: I = IO pm, w = 40 mm Y 9OGHz b 18VCiHz

Peak quiet zone ripple due to edge misalignment and tape pi l -

0 -2- :'i , , , , , , , , -5 -6 -400 -300 -200 -100 0 100 200 300 400

distance, m m Fig. 9 90 GlIz

~ Theoretical predlctlon

Quiet zone along a horizontal scan of the Q M W CATR at

Measurcments

material and the tape was stretched as much as it was practically possible flat, across the gap. Here a value of b = 20 p n has been included in the calculations pri- marily as a result of surface pillowing of the substrate over which the tape was fixed.

4 Pattern measurements

As already stated, a tape on the surface of the CATRs reflector induces clearly visible spurious lobes in the meas- ured pattern of an antenna at millimetre waves. In the simplified analysis reported in Reference 3 the strength of the spurious lobes relative to the main beam has been found using the following expression

where

R = distance between the tape and the test antenna

D = angle where the tape is seen from the test antenna position measured from the CATRs axis

t , w = thickness and the width of the tape

11Y

sinc (x) = sin nx/(nx)

It has been shown previously (Fig. 3 4 that the strength of the spurious lobes can be made very small when the tape is very thin, perfectly fixed and sufficiently displaced from the reflector's centre. So in principle the tape treat- ment has the potential of providing an effective solution even at 180 GHz. Here a more detailed investigation will be conducted. Again a measurement scenario of a two panel CATR with geometrical dimensions similar to the one installed at QMW [l] is assumed, with the test antenna placed so as to face the centre of the range reflec- tor, with the freedom of displacement along the axis of the range only. Millimetre wave operation at 180GHz will be considered here. Initially a tape perfectly fixed over a 3 mm wide gap with no misalignment present is assumed.

The worst case, as far as pattern accuracy is con- cerned, is when the tape is placed directly in the centre of the range reflector. Then the peak of the tape scattered field will interfere with the main beam characteristics of the test antenna. A typical result for 180 GHz operation can be seen in Fig. 10. Here a 250 mm test antenna aper-

-3 -2.4 -1.8 -1.2 -0.6 0 0 6 1.2 1.8 2.4 3 theta. degrees

Fig. 10 Tape at centre of reflector (w,*- = 40") t = 10 pm, w = 40mm, R = 6 0 o m m Polarisation: H Aprture size 250 mm, range ET: - 3 dB _ ~ _ _ Ideal {Taylor: n = IO, SLL = -65 dB) ~ Simulated measurement

Pattern deformation due to tape scattering at 180 GHz

ture is placed R = 6000 mm away from the tape. A com- prehensive selection of results concerning the peak level of the spurious response S (relative to the ideal response at boresight) induced on the pattern of the test antenna which faces the tape is seen in Fig. 11 . A 250 mm aper- ture operating at 90GHz placed at various positions along the CATRs axis, and tapes with different widths and thickness is assumed. Predictions using expression 7 are also included. The good agreement between the two methods, evident in Fig. 11, justifies the usage of eqn. 7 for estimating the effects of the tape scattering in relation with the testing of a physically small antenna at milli- metre waves.

As previously stated the monotonic width dependence seen in Fig. 11 is only confined to this, tape-test antenna, relative position. If the tape is displaced from the centre of the reflector the spurious lobe moves in the pattern and the angle where it emerges is determined from the new relative angular position of the tape as seen from the test antenna position measured from the range's axis. The level of the spurious lobe is determined from the quiet zone field characteristic in front of the test antenna,

I20

which, in turn, is affected by the detailed structure of the directional scattering pattern of the tape. As a result the monotonic dependence of spurious lobe strength and the

40

m 20 U

0 20 40 60 80 100 thickness.pm

-40 -30L I

Fig. 11 Spurious lobe leuel S when test apertures faces a tape placed on centre of C A T R reflector, derived from pattern measurements of a 250 mm aperture at 90 GHz Polarisation: H 0-0 Simulations x-x Calculations (eqn. 7) a w = 160 mm b w = W m m c w = 4 0 m m d w = IOmm

tape width is no longer true. It can be demonstrated that tapes with drastically different widths produce spurious lobes with comparable strengths.

The previous discussion assumes a tape perfectly fixed without including the impact of the gap underneath. Panel edge misalignment and tape pillowing have been shown to affect the quiet zone performance. These pertur- bations also affect the strength of the spurious effects recorded during pattern measurements. If the imperfec- tions considered in Fig. 7 are present during pattern measurement of a 250 mm test antenna then the pattern is corrupted by a spurious lobe with a progressively increasing strength, as shown in Fig. 12. It can be seen

theta, degree Fig. 12 Eflects of panel misalignment and tape pillowing during pattern measurements of a 250 mm aperture a( 180 GHz and R=6(MOI?Wl Polarisation : H T a p at o , . ~ = 45" with I = 10 pm, w = 40 mm _ _ _ _ Ideal (Taylor: n = IO, SLL = -65 dB) ~ Simulated measurement with: a m = b =Opm b m = M ) p m , b = O p m c m - 6 0 p m . b = 1 5 0 p m d Bare gap, width = 3 mm, rn = 60 pm

that the spurious lobe has increased by as much as 20 dB as a result of the combined action of edge misalignment and pillowing. The new situation is only marginally better than the untreated gap case. If the possible tape

IEE Proc.-Microw. Antennas Propag., Vol. 141, No. 2, April 1994

deformation, or misalignment, is assumed to have peak values, such as those selected in Fig. 12 (curve c), then the spurious lobe induced during the testing of the above antenna will have a strength laying within the bounds a and b shown in Fig. 13. A tape having the same charac-

curve of Fig. 9. Suppression of the spurious lobes by an average amount of 13 dB as compared to the case of a bare gap is observed.

When a larger antenna i s tested the spurious forma- tion has a smaller peak strength but larger angular extent, similar to the observations made when studying the bare gap case [l]. In Fig. 15 simulated pattern meas-

\I I - x

-251

_ _ 0 5 IO 15

spurious lobe angle.degree

Fig. 13 180 G H z a Upper envelope. tape with r = 10 p, w = 40 mm, m = 60 pm, h = 150 pm h Lower envelope tape with t = 10 e, w = 40 mm, m = b = 0 pm x

Spurious lohe envelope a/er testing a 25Omm aperture at

Tape with I = BOpm, w = 40 ptn, m = h = Opm

teristics as previously ( t = 10 pm, w = 40 mm) is assumed. It is interesting to note that an otherwise per- fectly applied 80pm thick tape of the same width pro- duces spurious lobes with strength generally weaker than a 10 pm thick tape but with deformations such as those selected for the upper bounding curve a shown in Fig. 13. Hence there is no point in selecting tapes with thickness very much smaller than the accuracy by which the panels are aligned or the tape can be fixed over the gap.

An E plane pattern measured in the QMW CATR is shown in Fig. 14. The test antenna has a 226 mm aper-

-9 -6 - 3 0 3 6 9 azimuth,degrees

Fig. 14 interpanel gaps at 186 GHz Aperture diameter: 200 mm _ _ _ _ theoretical predictions (tape treatment) ~ measurements ( t a p treatment)

Pattern measured in a panelled C A T R with tapes covering the

. . measurements (bare gaps)

ture and is placed so as to face the centre of panel 11 at the same distance where the quiet zone shown in Fig. 9 has been measured. The frequency of operation is 186 GHz, and the tape induced spurious lobes are clearly visible. Also shown are the spurious lobes predicted from the theory taking into account the same tape deforma- tion and misalignment as in the case of the theoretical

I E E Proc.-Microw. Antennas Prnpag.. Vol. 141, No. 2, April IYY4

-50 n E -60

-70 - 80 -90

-100

0

-2 -1 0 1 2 3 4 5 6 7 8 9 10 theta, degrees

Spurious effects induced on the pattern of U lOOOmm test Fig. 16 uperture due to tupe scattering at 180 C H z Polarisation: H Tape at qaW = 45' with t = 10 ptn, w = 40 mm, range ET: - 3 dB

~ ~ Ideal (Taylor: n = LO, SLL = -65 dB)

~ Simulated measurement (m = b = 0 pm) ~ Simulated meiwurement (m = 30 p, b = 30 p)

urements of a lo00 mm test aperture operating at 180 GHz is shown. A tape 10 pm thick and 40 mm wide is covering a 3 mm wide gap. The presence of tape. pil- lowing and panel misalignment is also taken into account to yield a representative pattern.

4 Conclusions

Tape treatment is a technique with the potential of reducing the effects caused due to diffraction arising at the interpanel gap edges of a panelled CATR at milli- metre waves.

Scattering produced from the tape induces spurious lobes in the pattern of a measured antenna. For a 10 pm thick tape perfectly fixed on the face of a panel the spurious effects are seen to have a very small strength (-52 dB) at 180 GHz. In practice, however, the effect- iveness of tape treatment will be determined by factors such as the panel size and geometrical arrangement, as well as the degree of panel alignment.

Tapes covering gaps in the central area of the CATR are likely to induce errors in the main beam or the near in sidelobes of the test antenna. Because of this a panel arrangement which avoids gaps in the central area of the CATR should be preferred. The larger the panel size the more the tape induced effects will be shifted away from the main beam. The shape of the tape's track on to the CATR reflector surface will inevitably be similar to the panel rim shape. In this sense the shape of the panel might be considered as an extra degree of freedom in order to suppress the tape induced effects. The underlying mechanism for such a potential improvement is the breaking up of the tape diffracted field coherency, and hence the spreading of the spurious lobe energy over an extended angular domain. To do so successfully the panel rim should have a sufficiently nonuniform shape over an area equal to the smallest test antenna aperture, expected to be tested. This approach, however, needs further investigation to establish firmly its applicability from the practical and financial point of view.

I21

The degree of alignment between adjacent panels is very important. The quantity usually given to speLlfy a panel quality is the RMS surface error. However, this value is not usually adequate to estimate the actual inter- panel edge misalignment. A picture of the surface error around the panel edges is also needed. Constraints then should be imposed on the value of the surface error allowed in this region. Under tape treatment a figure of panel edge misalignment of about 1./60 will result in spurious lobes with strength near to -50 dB for a mea- surement scenario similar to the one previously described.

The amount of tape pillowing is generally propor- tional to the width of the gap which the tape covers. Hence panels manufactured accurately enough to allow for a gap width around 1 mm or smaller should be used.

Experimentally spurious lobes with a level of around -45 dB have been observed when a 10 pm thick tape covered a 2mm wide gap. This figure is about 7dB higher, than the value that would be expected as a result of scattering from its edges only. Tape deformation and edge misalignment are contributing to this discrepancy. On the other hand this level of spurious lobe represent an average 13 dl3 improvement relative to the bare gap case.

5 References

I PHILIPPAKIS, M., and PARINI, C.G.: ‘Two dimensional model- ling of antenna pattern measurements in a panelled compact range

at millimetrewaves’. Proceedings of the IEEE Antennas and Propa- gation Society international symposium (IEEE-APS), Chicago, IL, 1992, pp. 317-320

2 GUPTA, LJ., and BlJRNSIDE, W.D.: ‘Scattered fields of metallic tapes used to cover the gaps in compact range reflectors’. Pro- ceedings of the Antenna Measurement and Techniaues Association (AMTA), 1989, pp. 15.35-15.39

3 PHILIPPAKIS, M., and PARINI, C . G . : ‘An experimental and theoretical study of some aspects concerning the performance of a millimetrewave compact antenna test range’. Proceedings of the international conference on Antennas and propagation (ICAP), York, UK, 1991, pp. 770-773

4 PHILIPPAKIS, M., and PARINI, C.G.: ‘Application of the parallel plane canonical problem in predicting the diffraction caused by a tape fixed on a ground plane’, Electron. Lett., 2% (15), July 1992, pp. 1413

5 SOMERS, G.A., and PATHAK, P.H.: ‘Uniform GTD solution for diffraction by metallic tapes on panelled compact range reflectors’, IEE Proc. H., June 1992,139, (31, pp. 297-305

6 McNAIR, P.A., OLVER, A.D., and PARINI, C.G.: ‘A millimetre wave compact antenna range’. ESTEC Workshop on Antenna Metrology,.1988, pp. 81-85 ~

7 NOBLE. E.: ‘Methods based on the Wiener Hoof techniaue’ (Chelsea Publications, 1988)

8 MITTRA, R., and LEE, S.W.: ‘Analytical techniques in the theory of guided waves’ (Macmillan, London, 1971)

9 McNAMARA, D.A., PISTORIOUS, C.W.I., and MALHERBE, J.A.G.: ‘Introduction to the uniform theory of diffraction’ (Artech House, 1990), Chap. 4

IO JULL, E.V.: ‘Radiation from apertures’, in LO, Y.T., and LEE, S.W. (Eds.): ‘Antenna handbook theory applications and design’ (Van Nostrand Reinhold. New York, 1988). pp. 5.18-5.20

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